1. Field of the Invention
The present invention relates to laser processing of materials such as semiconductor redundant memory links and, in particular, to a laser system and method utilizing a single laser pulse having an ultra short pulse width to process target material such as to sever a memory link. The present invention is also applicable to other laser-based micro-machining and repair operations. For example, the invention may be adapted for removing microscopic target material while avoiding undesirable damage to adjacent non-target material(s) having a thermal or optical property different than the target material.
2. Background Art
The following exemplary publications are related to micro-machining with ultrashort laser pulses:                1. Kruger et al., “Femtosecond-Pulse Laser Processing of Metallic and Semiconducting Thin Films,” PROC. SPIE, Vol. 2403, pp. 436–447, 1995.        2. Chichko et al., “Femtosecond, Picosecond, and Nanosecond Laser Ablation of Solids,” APPLIED PHYSICS, A 63, 109–115, 1996.        3. Stuart et al., “Ultrashort-Pulse Optical Damage,” PROC. SPIE, Vol. 2714, pp. 616–629, 1996.        4. Haight et al., “Implementation and Performance of a Femtosecond Laser Mask Repair System in Manufacturing,” pp. 1–8, IBM, 1998.        5. Islam et al., “On Ultrashort Laser Pulse Machining,” TECHNICAL REPORT, 1998.        6. Zhao et al., “Micromachining with Ultrashort Laser Pulses,” PROC. SPIE, Vol. 3618, pp. 114–121, 1999.        7. Zhu et al., “Influence of Laser Parameters and Material Properties on Micro-Drilling with Femtosecond Pulses,” APPLIED PHYSICS A69 [Suppl], 367–S371, 1999.        8. Tonshoff et al, “Micromachining Using Femtosecond Lasers,” LPM 2000 CONFERENCE, June 2000.        9. Perry et al., “Ultrashort-Pulse Laser Machining,” LIA HANDBOOK OF LASER MATERIALS PROCESSING, Ed. In Chief Ready, Laser Institute of America, pp. 499–508, 2001.        10. Perry et al., “Ultrafast Lasers for Material Processing,” LIA HANDBOOK OF LASER MATERIALS PROCESSING, Ed. In Chief Ready, Laser Institute of America, p. 82, 2001.        
Reference 4 to Haight et al. specifically relates to application of ultrashort technology for repair of chromium defects on a photomask, an example of a laser repair application in microelectronics.
The following exemplary U.S. patents and published applications relate to micromachining with ultrashort laser pulses:                1. U.S. Pat. No. 5,656,186, “Method for Controlling Configuration of Laser Induced Breakdown and Ablation.”        2. U.S. Pat. No. 6,285,002, “Three Dimensional Micro-Machining with a Modulated Ultra-short Laser Pulse.”        3. U.S. Pat. No. 6,286,586, “Method and Apparatus for Improving Quality and Efficiency of Ultrashort-Pulse Machining.”        4. U.S. Pat. No. 6,333,485, “Method for Minimizing Sample Damage During Ablation of Material Using a Focused Ultrashort Pulsed Beam.”        5. Published U.S. patent application Ser. No. 2002/0003130, “Laser System and Method for Processing a Memory Link with a Burst of Laser Pulses Having Ultrashort Pulse Widths.”        
The '485 patent specifically relates to application of ultrashort technology for repairing chromium defects on a photomasks, an example of a laser repair application in microelectronics. The '3130 published application is related to semiconductor laser memory repair.
Exemplary specifications for next generation DRAM devices include link widths less than 0.5 microns and the link pitch (spacing) less than 2 microns (e.g., 1.33 microns or as fine as about 1 micron). Current commercial laser memory link repair systems, which typically use Q-switched, Nd: based solid state lasers with wavelengths of 1 to 1.3 microns and pulse widths 4 to 50 nsec, are rapidly approaching limits. The large spot size due to the wavelength used and thermal effect due to the pulse width used are two limiting factors.
Solutions using short wavelength lasers have been proposed, for instance green (e.g.: 532 nm wavelength typical) and solid state UV lasers (e.g.: 355 nm, 248 nm, and shorter). U.S. Pat. Nos. 6,057,180 and 6,025,256 describe methods of using a nano-second UV laser to sever the links. Although the short wavelength has the benefit of a smaller beam spot size, the relative long pulse width of these conventional lasers makes the thermal process dominant. The chances of Silicon substrate damage at the shorter wavelengths greatly increases. Further, the size and the pitch of the links to be processed become limited as a result of neighbor link damage. The tolerance budget of the laser system as a whole must be considered so as to produce acceptable yields at the fine pitch scale. Material modifications and/or the introduction of shielding layers have been proposed, for instance as disclosed in U.S. Pat. Nos. 6,057,180; 6,297,541; and 6,664,163 and EP patent application No. 0902474. Approaches have been proposed to reduce collateral damage (adjacent link damage).
U.S. Pat. No. 5,656,186 discloses a general method of laser-induced breakdown and ablation by ultra fast laser pulses. It describes the dependence of the fluence threshold on laser pulse width. The slope of such dependence will show a rapid change from the typical square root relationship when the pulse width is less than the so-called breakdown point pulse width. For metals, the breakdown point is typically around 10 ps, but may vary from an exemplary range of hundreds of femtoseconds to tens of picoseconds.
U.S. Pat. No. 5,208,437 discloses the use of a single pulse to process the link with a pulse width less than 1 nsec. However, the existence of the break down point, as indicated in U.S. Pat. No. 5,656,186, is not disclosed.
Numerous references teach using ultra fast lasers for micro-machining at low pulse energies, low peak energy densities, and low peak power densities that are close to ablation threshold. The etching rates per pulse are comparable to the depth of the optical absorption.
An application of ultrashort processing to link blowing is disclosed in published U.S. patent application Ser. No. 2002/000313. Links are to be processed by applying a pulse train of ultra short laser pulses to each link at very high repetition rates. Etching rates per pulse are in the range of 0.01–0.03 microns per pulse for link material removal, and typically between 0.01 to 0.2 microns per pulse to remove a layer of passivation material. Multiple pulses or pulse trains are required to sever a link that is 0.05 microns or thicker. Therefore, multiple pulses are also needed to remove the memory redundancy link that is more than 0.4 micron thick.